1. Technical Field
The present invention relates to secure optical data communications and, in particular, to providing secure frequency-hopping in optical OFDM systems.
2. Description of the Related Art
Optical fiber systems form the backbone of global communication networks. As groups such as financial institutions, large corporations, and militaries demand greater security for their communications, providing additional security for the physical layer becomes important. Securing the physical layer may be performed alongside measures taken to secure higher network layers, such as encryption.
It is possible to tap an optical fiber to extract the transmitted information. One way to accomplish this is simply to bend the fiber past the point where all light is internally reflected, allowing some of the light to leak out. Although it was possible in the past to detect fiber tapping by noting an increase in signal attenuation, modern tapping techniques allow for tapping without significantly increasing the attenuation. However, even if the tapping is detected, this may be too late, as information has already been accessed.
One previous attempt to address this concern came in the form of orthogonal code division multiple access (CDMA), where digital bit information is encoded with a specially designed set of codes. At the receiver side, only users which have the knowledge of the codes can correctly decode the received signal and recover the transmitted bit information. Although CMDA is popular in wireless communications, optical CDMA has limited applications in optical communication networks.